In control mice of different ages (including senile mice of long-lived strains) and in mice on caloric undernutrition (Mackay diet) functional status of the immune system is being assessed, with particular emphasis now upon "synergy" or the ability of T cell subsets to collaborate, upon the increase in suppressor cells for the mixed lymphocyte reaction with age, and upon the delineation of thymus cell subsets. The effect of thymic hormonal factors and of certain other reagents such as polynucleotides on the immunodeficiency of aging are under investigation, with particular reference to their influence on cyclic AMP and cyclic GMP levels in lymphoid tissues of older mice. BIBLIOGRAPHIC REFERENCES: Gerbase-DeLima, M., Meredith, P., and Walford, R.L.: Age-related changes, including synergy and suppression, in the mixed lymphocyte reaction in long-lived mice. Fed. Proc. 34: 159-161, 1975. Liu, R.K., and Walford, R.L.: Mid-life temperature-transfer effects on lifespan of Annual fish. J. Gerontology 30: 129-131, 1975.